Fourier Transform Infrared Spectroscopic Study of the Interactions of a Strongly Antimicrobial but Weakly Hemolytic Analogue of Gramicidin S with Lipid Micelles and Lipid Bilayer Membranes

Abstract

Cyclo[VKLdKVdYPLKVKLdYP] (GS14dK₄), a synthetic tetradecameric ring-size analogue of the naturally occurring antimicrobial peptide gramicidin S (GS), retains the strong antimicrobial activity of GS but is 15−20 times less hemolytic. To characterize its interaction with lipid membranes and to understand the molecular basis of its capacity to lyse bacterial cells, in preference to erythrocytes, we have investigated the interactions of GS14dK₄ with detergent micelles and with lipid bilayer model membranes by Fourier transform infrared spectroscopy and compared our results with those of a similar study of GS [Lewis, R. N. A. H., et al. (1999) Biochemistry 38, 15193−15203]. In both aqueous and organic solvent solutions, GS14dK₄ adopts a β-sheet conformation that is somewhat distorted and more sensitive to the polarity of its environment than GS. Like GS, GS14dK₄ is completely or partially excluded from gel-state lipid bilayers but interacts strongly with liquid-crystalline lipid bilayers and detergent micelle, and interacts more strongly with more fluid liquid-crystalline lipid systems. However, its interactions are more strongly influenced by membrane lipid order and fluidity, and unlike GS, it is essentially excluded from cholesterol-containing phospholipid bilayers. Also, GS14dK₄ is excluded from cationic lipid bilayers, but partitions more strongly and/or penetrates more deeply into anionic lipid bilayers than into those composed of either zwitterionic or nonionic lipids. Anionic lipids also facilitate GS14dK₄ interactions with multicomponent lipid bilayers which are predominantly zwitterionic or nonionic. Although GS14dK₄ generally penetrates and/or partitions into zwitterionic or uncharged lipid bilayers less strongly than does GS, its greater size and altered distribution of positive charges make it intrinsically more perturbing with regard to membrane organization once associated with lipid bilayers. This fact, combined with its relatively strong interactions with anionic phospholipids, may explain why GS14dK₄ retains relatively high antimicrobial activity. However, its low hemolytic activity is probably largely attributable to its low propensity to penetrate and/or partition into cholesterol-containing zwitterionic lipid membranes.